Many frequency controlled tunable circuits, such as oscillator or filter circuits, have non-linear frequency output characteristics, that is, the frequency output vs. control (current or potential) characteristic is not always linear. When good linearity is desired, oscillator designers typically select component types and apply resonant effects to achieve a characteristic that meets the requirement as nearly as possible. Sometimes it is not possible or practical to achieve the desired linearity and system designs need to be modified. Even when linearity is achievable, component and dimension tolerances can require either that components are individually selected following device tests, or that physical adjustments are made during the manufacturing process. This obviously increases production costs.
In some applications, absolute control rate can be important. Temperature dependencies can limit the final system performance, or require that an otherwise low-power system be placed in a temperature controlled environment, such as an oven.
A constant frequency tuning rate can be important for systems such as temperature compensated crystal oscillators, to avoid degradation of frequency accuracy when the oscillator is tuned away from the conditions under which it was compensated. Such tuning may be necessary to correct for, for example, ageing of the quartz crystal, or to match the operating environment. One situation where this is particularly relevant is in retiming circuits where a local crystal oscillator tracks an intermittent input clock with a frequency that is allowed to deviate from a nominal value. To ensure a rapid recovery from loss of input clock signal, the local crystal oscillator is required to continue oscillating at the last observed frequency for extended periods.
Communication, navigation, and timing systems often need to synchronise their local frequency sources to a remote reference. It is helpful for the control of this synchronisation for the tuning rate of the local source to be constant—i.e. linear and independent of temperature. These same characteristics are beneficial for maintaining constant frequency output from temperature compensated oscillators after the oscillator has been re-tuned. Suitable resonators for such oscillators can include acoustic devices such as bulk-mode crystals and SAWs, dielectric resonators such as ceramic pucks and cooled sapphire and hybrid arrangements.
There is thus a need to provide for a more linear tuning rate for such systems, and/or for the tuning rate to be matched for individual oscillators or resonators and/or to provide compensation for typical temperature variations in oscillator output.
In GB 2369259 some tuning linearity and temperature corrections can be achieved using a circuit which provides a predistorted control signal. While the method of predistortion in GB 2369259 was capable of providing tuning characteristics with excellent linearity and temperature independence, it can prove difficult to maintain fast response and low noise while minimizing the circuit's dissipation. The present invention can provide an alternative and improved way of achieving linearity and temperature correction.